Method for development of electrostatic images



March 10, 1959 E. P. MATTHEWS 2,

METHOD FOR DEVELOPMENT OF ELECTROSTATIC IMAGES Flled Feb. .18, 1955 2Sheets-Sheet 1 SENSITIZATION TRANSFER T (CHARGING) EXPOSURE DEVELOPMENOR FIXING POWDER CLOUD CHARGING POWDER CLOUD GENERATOR \O l2 l4 13 llFIGB lNVENTOR EARNEST PAUL MATTHEWS ATTORNEY March 10, 1959 E. P.MATTHEWS 2,877,132

METHOD FOR DEVELOPMENT OF ELECTROSTATIC IMAGES Filed Feb. 18, 1955 2Sheets-Sheet 2 FIG. 4

\ POWDER I POWDER CLOUD CLOUD GENERATOR CHARGING lNVENTOR EARNEST PAULMATTHEWS I ATTORNEW United States Patent METHOD FOR DEVELOPMENT OFELECTROSTATIC IMAGES Earnest Paul Matthews, Alliance, Ohio, assignor, bymesne assignments, to Haioid Xerox Inc., Rochester, N. Y., a corporationof New York Application February 18, 1955, Serial No. 489,083

4 Claims. (Cl. 117-175) This invention relates to a method and apparatusfor the development of electrostatic latent images.

In xerography it is usual to form an electrostatic latent image on asurface. One method of doing this is to charge a photoconductive,insulating surface and then dissipate the charge selectively by exposureto a pattern I of activating radiation. Other means of formingelectrostatic latent images are set forth in U. S. 2,647,464 to James P.Ebert. Whether formed by these means or any other, the resultingelectrostatic charge pattern is conventionally utilized by thedeposition of an electroscopic material thereon through electrostaticattraction whereby there is formed a visible image of electroscopicparticles corresponding to the electrostatic latent image.Alternatively, the electrostatic charge pattern may be transferred to aninsulating film and the electroscopic particles deposited thereon toform the visible image. In any case, this visible image, in turn, may betransferred to a second surface to form a xerographic print.

The process of depositing the electroscopic powder on the electrostaticimage to render the electrostatic image visible is called thedevelopment step and is one of the most critical steps of the entireprocess. The step is of particular importance both on machines designedfor continuous operation with any type of copy and in processingcontinuous-tone images. Now, in accordance with the present invention, amethod and an apparatus are provided for substantially improveddevelopment of an electrostatic latent image.

Fig. 1 of the attached drawings is a block diagram showing the positionof the development step in an overall xerographic process which resultsin a visible image.

Fig. 2 is a diagrammatic side elevation in cross sec tion of apparatusaccording to one embodiment of the invention showin gthe initial step indevelopment using this embodiment.

Fig. 3 is a diagrammatic side elevation in cross section of the sameembodiment showing the second step in development.

Fig. 4 is an isometric drawing of the same embodiment with a cutawaysection.

Fig. 5 is a diagrammatic side elevation in cross sec- 'tion of apparatusaccording to another embodiment or the invention.

Fig. 6 is an isometric drawing of the embodiment shown in Fig. 5 with acutaway section.

As shown in Fig. 1, the general xerographic process involves theformation of an electrostatic latent image. This is generally, althoughnot always, preceded by a treatment to sensitize the surface on whichthe electrostatic image is to be formed. The electrostatic latent icevclopment step permits. About the coarsest type of image reproduced by axerographic process requires a resolution of at least about 50 lines perinch. Commercial, line-copying machines generally have a resolutionpower of about to 250 lines per inch. The process used in obtaining thisresolution is set forth in U. S. 2,618,552 and involves the use of afinely-divided, colored material called a toner deposited on a slightlymore coarsely-divided material called a carrier. This twocomponentdeveloper is cascaded across the electrostatic latent image areas. Thecontrol of the concentration of toner in the two-component developerbecomes extremely diificult in thecontinuous operation of such line-copymachines. When applied to continuous-tone development where resolutionsof about 50 or more lines per millimeter are often desired, it has beenfound impossible to obtain this high quality of reproduction using sucha system. Accordingly, a system known as powder cloud development ispreferred. This is the system incorporated in Fig. 1.

As shown in Fig. 1, a powder cloud is generated. The cloud so producedis then charged, the charging step constituting either a separate stepor utilizing the inherent charge on the cloud in the case of a cloud ofdry particles. A cloud so generated and charged is then contacted withthe electrostatic latent image in the development step. The visibleimage so produced may be used as such, permanently afiixed to the plate,or many be transferred to another material as a sheet of paper orplastic, as is well-known to those skilled in the xerographic art.

The instant invention relates to a process and apparatus therefor whichrepresent a substantial improvement in the art of powder clouddevelopment. Many of the methods of powder cloud development usedheretofore have been characterized by various difficulties, thus: Whenthe powder cloud in its flow travels over an area that should be whiteand then over an area that should be dark, the leading edge of the darkarea is not developed. On the developed image the dark area appears tobe torn so that a white background shows through. Because of thisappearance the flaw is referred to as tearing. Another difiiculty occurswhen the powder cloud travels over a large, dark area and then over alarge area that should remain white. Although the white area is void ofcharge, it does not remain free of powder. Developer particles depositin streaks through this area in amounts roughly proportional to thelength of the dark area that immediately precedes the white area. Thisdifiiculty is termed streaking. Still another difficulty is thatdeveloped images are not uniformly developed end-to-end nor doesextending the development time remedy this defect, as apparently part ofthe charge in the image areas at one end of the plate are erased in thedevelopment process. The process of the instant invention representssubstantial improvement on all these. In general, the present inventionaccomplishes these objectives by reversing the flow of the powder cloudover the image areas during the development step. The proc ess of theinvention and apparatus for its accomplishment will now be set forth inmore detail.

According to the present invention, a cloud of electroscopic particlesis generated in a suitable cloudforming device; for example, as byagitation of a powder mass in a closed container with a rotating brush,as in U. S. Patent No. 2,357,809 to C. F. Carlson, or a rotating airmass as caused by laterally-located nozzles. If a cloud of liquiddroplets is desired, it may be produced by any means known to thoseskilled in the art as by spraying through an atomizer. The cloud bywhatever means produced is then charged. Where a cloud of dry particlesis used, any method of generating the cloud will almost necessarilyproduce a charge thereon. For some purposes, the charge so produced willbe adequate. If it is desired to produce a more uniform charge, however,other devices, such as triboelectric charging (as by passing theelectroscopic powder through anarrow tube of suitable material inturbulent flow), corona charging or other charging device, may be used.In the case of a cloud of liquid droplets, charging means such asinduction charging, corona charging, and so forth, may be used. Thecharged cloud is then optionally passed into a collecting zone and thenthrough suitable valving means into the development zone alternatelyfrom each end.

The development zone is the area between the development electrode andthe'image-bearing surface. The development electrode draws the field offorce of the image externally'above the image-bearing surface. To dothis, the development electrode should be no further from theimage-bearing surface than Mr" and, desirably, should be no further than.4:". For high quality resolution, the spacing should be no more than ,4course extend over the entire image surface. The charged electroscopicparticles are passed over the imagebearing surface within the field offorce. Passing the powder cloud once from each end through thedevelopment zone is sufiicient to realize the advantages of the instantprocess. If desired, however, the direction of flow may be alternatedseveral times in the course of development with some improvementresulting from the greater number.

The general nature of the process of the invention having been setforth, specific embodiments of the invention will now be described. Theinvention, however, is not limited to these embodiments which arepresented necessarily for purposes of illustrating means of working theinstant invention.

As shown in Fig. 2, the apparatus illustrated contemplates a supportplate having entrance holes 13 and 14 and exit holes 11 and 12.Superimposed on the support plate in movable relation thereto is a baseplate 15 also having entrance holes 18 and 19 and exit holes 16 and 17.Positioned thereon are spacing means 20 defining left and right chambers21 and 22. 7 On top of the spacer is the development electrode 23 havingslots therein, 24 and 25, which may serve as either entrance or exitslots. Positioned closely adjacent to the development electrode by meansof spacers 26'is an electrostatic image- 'bearing member comprising aphotoconductive insulating layer 29 coated on a conductive backing 27.Stop plate 28 aligns the assembly.

In operation, as shown in Fig. 2, which depicts the first step ofdevelopment, the powder cloud flows through entrance slots 13 and 18into right chamber 22 where it expands and then flows through slot 24into the development zone 48 between the development electrode and theimage-bearing member. leaves by means of slot where the cloud passesinto chamber 21 and then through the exit slots 17 and 12. This positionis maintained for no more than one-half the development cycle.

Then the assembly is shifted as shown in Figure 3. This may be done byholding the support plate stationary and moving the rest of the assembly(as was done here), or by moving the support plate (which requiresflexible tubing to carry the powder cloud to and from the apparatus) andholding the rest of the assembly stationary. In this position the powdercloud now enters through entrance holes 14 and 19 into chamber 21 wherethe cloud fans out, flows through slot 25 into development zone 48,leaves the development zone through slot 24, enters chamber 22, andleaves through exit holes 16 and 11. This cycle may be repeated as oftenas desired in the course of the development of the electrostatic image.By this means the'direction of flow of the powder cloud with respect tothe xerographic 'plate being developed may be reversed as often asdesired in the course 'of development.

It should, of

Passing through this zone, it

ase'a se H In this embodiment of apparatus according to the inventionthe left and right chambers 21 and 22 afford additional advantagesbeyond those resulting from the process of alternate development fromeach end of the development zone. Thus these chambers provides asmoother and more uniform powder cloud as it is introduced into thedevelopment zone with resultant improvement in print quality. Inaddition, the chambers in combination with the slot exhaust promotes amore uniform development.

Fig. 4 shows an isometric view of the same development apparatus.

Fig. 5 shows another embodiment of the invention wherein valving meansare substituted for physical move ment of surfaces.

In the apparatus shown, the powder cloud is generated 42, charged 46 andfed to a two-way valve 31 by a tube 43. The valve diverts the powdercloud to one of the two exit tubes, say, the right-hand tube 3. Thepowder cloud flows through this tube to the entrance slot 44 where theair pressure exerted by the cloud forces up gate means 32 therebypermitting the powder cloud to flow through the development zone 33 inroughly laminar flow leaving through exit means 34. When no more thanhalf the development time has elapsed the valve position is changedsending'the powder cloud through the left-hand tube 35 thence to theentrance slot 45 where, by means of the pressure exerted by the powdercloud, gate means 36 are forced up permitting the powder cloud to flowthrough the development zone in approximately laminar flow leavingthrough exit means 37. In the apparatus shown, 38 is the developmentelectrode and 39 the xerographic plate having an image-bearing surface40. Spacing means 41 are provided to position the xerographic plate atthe correct distance from the development electrode, namely, no morethan about%" apart.

Fig. 6 shows an isometric view of the same development apparatus.

The method of the instant invention may also be used on a continuousmachine of the rotating-drum type. In this apparatus a photoconductiveinsulating layer is coated on the curved surface of a cylindrical drum.The various steps of the overall xerographic process as shown in Fig. lare carried out around the periphery of the curvedsurface as the imageis moved from step to step by rotating the drum on its lengthwise axis.When using the method of the instant invention, the drum is rotated inone direction (as clockwise) but stepwise rather than at a constant,.uniform speed. Thus, a given arc of the drums periphery, say fourinches, is advanced in one step and the drum stopped for a set period,say three seconds, to permit development, then the step is repeated.

On whatever type of machine the process of the invention is used, apreferred method of operation is to use alternate development periods ofshort duration. For example, if in a total development time of 4 secondsthe image is developed for 2 seconds in each direction, tearing andstreaking will be definitely reduced. However, if development were forperiods ofJ/z second from each direction alternately, a much morecomplete covering of tears and streaks would be achieved. Hence, for anygiven overall development period it is preferred to reverse thedirection of powder cloud flow through the development zone almost asfrequently as the equipment permits within the available time.

Where the electrostatic image is borne by a photoconductive insulatinglayer coated on a conductive backing as in the embodiments shown inFigs. 2, 3 and 5, desirably. means are provided for applying an electricpotential difference between the conductive backing member 27 for thephotoconductive insulating layer 29' and'the development electrode 23.These means may include, for example, as shown in Fig.2, a battery 46 orsimilar D. C. power source connected through a potentiometer 47 to, inthis case, the conductive backing 27,. The conductive backing andphotoconductive insulating layerare,

of course, insulated from the development electrode in such a system andthe electrode itself is grounded as shown. If desired, the potential maybe applied to the development electrode and the conductive backingmember grounded.

Generally, the powder cloud is charged with a charge of polarityopposite to the polarity of the electrostatic image. However, this isnot necessarily so. Thus, reversal development may be efiected bycharging the powder cloud with a charge having the same polarity as theimage areas. Hence, the powder is repelled from the image. Reversaldevelopment may also be obtained by use of the proper bias frompotentiometer 47 between the development electrode 23 and the backingmember 27. In this system, the potentiometer is adjusted to bias theconductive backing member relative to the development electrode to thehighest voltage on the photoconductive insulating layer 23 and of theopposite polarity to the electrostatic image. Where the potentiometer isconnected to the electrode and the conductive backing member isgrounded, the potential is of the same polarity as the electrostaticimage. In either case, the field applied opposes the image field. Thepowder (charged to a polarity opposite to that to which thephotoconductive insulating layer was initially charged) is attracted tothe uncharged background areas to develop a reversal print.

Other modifications and variations of the basic invention will, ofcourse, be obvious to those skilled in the art. What ever variation ofthe basic process is used, however, it is possible by this process tosubstantially reduce tearing and streaking while greatly improvingend-to-end uniformity of the developed xerographic image.

I claim:

1. A process for developing an electrostatic image on an image-bearingsurface, said process comprising drawing the field of force of saidimage externally above the image-bearing surface, generating andelectrostatically charging a cloud of powder particles, and passing thepowder cloud along the image-bearing surface within the field of forcealternately from each of two opposite ends of the image-bearing surface.

2. A process for developing an electrostatic image on an image-bearingsurface, said process comprising drawing the field of force of saidimage externally above the imagebearing surface, generating andelectrostatically charging a cloud of powder particles, and passing thepowder cloud along the image-bearing surface within the field of forcemore than once from each of two opposite ends of the image-bearingsurface.

3. A process for developing an electrostatic image on an image-bearingsurface, said process comprising drawing the field of force of saidimage externally above the image-bearing surface, generating andelectrostatically charging a cloud of powder particles, passing thepowder cloud along the image-bearing surface within the field of forcefrom one end of the image-bearing surface for about one-half of thedevelopment time, then reversing the flow of the powder cloud andpassing the powder cloud along the image-bearing surface within thefield of force for the remainder of the development time from theopposite end of the image-bearing surface.

4. A process for developing an electrostatic image on an image-bearingsurface, said process comprising positioning said image-bearing surfaceat a distance of no more than about /4 inch from a conductive electrodeto form a development zone, applying a potential to the said electrodeequal to the highest potential on the imagebearing surface and of thesame polarity as the image, generating and electrostatically charging acloud of powder particles, passing the powder cloud along theimage-bearing surface within the development zone alternately from eachof two opposite ends of the image-bear ing surface.

References Cited in the file of this patent UNITED STATES PATENTS Re.22,419 Smyser Jan. 11, 1944 2,550,724 Sabel et al. May 1, 1951 2,618,552Wise Nov. 18, 1952 2,659,670 Copley Nov. 17, 1953 2,684,656 RansburgJuly 27, 1954 2,711,481 Phillips June 21, 1955 2,716,826 Huebner Sept.6, 1955 2,725,304 Landrigan et a1. Nov. 29, 1955

1. A PROCESS FOR DEVELOPING AN ELECTROSATIC IMAGE ON AN IMAGE-BEARINGSURFACE, SAID PROCESS COMPRISING DRAWING THE FIELD OF FORCE OF SAIDIMAGE EXTERNALLY ABOVE THE IMAGE-BEARING SURFACE, GENERATING ANDELECTROSTICALLY CHARGING A CLOUD OF POWDER PARTICLES, AND PASSING THEPOWDER CLOUD ALONG THE IMAGE-BEARING SURFACE WITHIN THE FIELD OF FORCEALTERNATELY FROM EACH OF TWO OPPOSITE ENDS OF THE IMAGE-BEARING SURFACE.